Download ppt - Logical and Bit Operations Chapter 9 S. Dandamudi

Logical and Bit Operations

Chapter 9S. Dandamudi

2005To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.

S. Dandamudi Chapter 9: Page 2

Outline

• Logical instructions AND OR XOR NOT TEST

• Shift instructions Logical shift instructions Arithmetic shift instructions

• Rotate instructions Rotate without carry Rotate through carry

• Logical expressions in high-level languages Representation of

Boolean data Logical expressions Bit manipulation Evaluation of logical

expressions• Bit instructions

Bit test and modify instructions

Bit scan instructions• Illustrative examples



Logical Instructions

• Logical instructions operate on bit-by-bit basis• Five logical instructions:

AND OR XOR NOT TEST

• All logical instructions affect the status flags



Logical Instructions (cont’d)

• Since logical instructions operate on a bit-by-bit basis, no carry or overflow is generated

• Logical instructions Clear carry flag (CF) and overflow flag (OF) AF is undefined

• Remaining three flags record useful information Zero flag Sign flag Parity flag




AND instruction• Format

and destination,source• Usage

To support compound logical expressions and bitwise AND operation of HLLs

To clear one or more bits of a byte, word, or doubleword

To isolate one or more bits of a byte, word, or doubleword




OR instruction• Format

or destination,source

• Usage To support compound logical expressions and bitwise

OR operation of HLLs To set one or more bits of a byte, word, or doubleword To paste one or more bits of a byte, word, or

doubleword




XOR instruction• Format

xor destination,source

• Usage To support compound logical expressions of HLLs To toggle one or more bits of a byte, word, or

doubleword To initialize registers to zero

» Example: xor AX,AX




NOT instruction• Format

not destination

• Usage To support logical expressions of HLLs To complement bits

» Example: 2’s complement of an 8-bit numbernot ALinc AL




TEST instruction• Format

test destination,source TEST is a non-destructive AND operation

» Result is not written in destination» Similar in spirit to cmp instruction

• Usage To test bits

» Example:test AL,1jz even_number ; else odd number



Shift Instructions

• Two types of shift instructions Logical shift instructions

» shl (SHift Left)» shr (SHift Right) » Another interpretation:

– Logical shift instructions work on unsigned binary numbers

Arithmetic shift instructions» sal (Shift Arithmetic Left)» sar (Shift Arithmetic Right)» Another interpretation:

– Arithmetic shift instructions work on signed binary numbers



Shift Instructions (cont’d)

• Effect on flags Auxiliary flag (AF): undefined Zero flag (ZF) and parity flag (PF) are updated to reflect

the result Carry flag

» Contains the last bit shifted out Overflow flag

» For multibit shifts– Undefined

» For single bit shifts– OF is set if the sign bit has changed as a result of the shift– Cleared otherwise



Logical Shift Instructions

• General formatshl destination,countshr destination,count

destination can be an 8-, 16-, or 32-bit operand located either in a register or memory



Logical Shift Instructions (cont’d)

• Two versions shl/shr destination,countshl/shr destination,CL

First format directly specifies the count value» Count value should be between 0 and 31» If a greater value is specified, Pentium takes only the least

significant 5 bits as the count value Second format specifies count indirectly through CL

» CL contents are not changed» Useful if count value is known only at the run time as opposed

at assembly time– Ex: Count is received as an argument in a procedure call



Logical Shift Instructions (cont’d)

• Usage Bit manipulation

; AL contains the byte to be encryptedmov AH,ALshl AL,4 ; move lower nibble to uppershr AH,4 ; move upper nibble to loweror AL,AH ; paste them together; AL has the encrypted byte

Multiplication and division» Useful to multiply (left shift) or divide (right shift) by a power of 2» More efficient than using multiply/divide instructions



Arithmetic Shift Instructions

• Two versions as in logical shiftsal/sar destination,countsal/sar destination,CL



Double Shift Instructions

• Double shift instructions work on either 32- or 64-bit operands

• Format Takes three operands

shld dest,src,count ; left-shiftshrd dest,src,count ; right-shift

dest can be in memory or register src must be a register count can be an immediate value or in CL as in other

shift instructions



Double Shift Instructions (cont’d)

• src is not modified by the doubleshift instruction• Only dest is modified• Shifted out bit goes into the carry flag



Rotate Instructions

• A problem with the shift instructions Shifted out bits are lost Rotate instructions feed them back

• Two types of rotate instructions Rotate without carry

» Carry flag is not involved in the rotate process Rotate through carry

» Rotation involves the carry flag



Rotate Without Carry

• General formatrol destination,countror destination,count

count can be an immediate value or in CL (as in shift)



Rotate Through Carry

• General formatrcl destination,countrcr destination,count

count can be an immediate value or in CL (as in shift)



Rotate Through Carry (cont’d)

• Only two instructions that take CF into account» This feature is useful in multiword shifts

• Example: Shifting 64-bit number in EDX:EAX Rotate version

mov ECX,4 ; 4 bit shiftshift_left: shl EAX,1 ; moves leftmost bit of EAX to CF rcl EDX,1 ; CF goes to rightmost bit of EDX loop shift_left

Double shift version shld EDX,EAX,4 ; EAX is unaffected by shld

shl EAX,4



Logical Expressions in HLLs

• Representation of Boolean data Only a single bit is needed to represent Boolean data Usually a single byte is used

» For example, in C– All zero bits represents false– A non-zero value represents true

• Logical expressions Logical instructions AND, OR, etc. are used

• Bit manipulation Logical, shift, and rotate instructions are used



Evaluation of Logical Expressions

• Two basic ways Full evaluation

» Entire expression is evaluated before assigning a value» PASCAL uses full evaluation

Partial evaluation» Assigns as soon as the final outcome is known without blindly

evaluating the entire logical expression» Two rules help:

– cond1 AND cond2 If cond1 is false, no need to evaluate cond2

– cond1 OR cond2 If cond1 is true, no need to evaluate cond2



Evaluation of Logical Expressions (cont’d)

• Partial evaluation Used by C

• Useful in certain cases to avoid run-time errors• Example

if ((X > 0) AND (Y/X > 100)) If x is 0, full evaluation results in divide error Partial evaluation will not evaluate (Y/X > 100) if X = 0

• Partial evaluation is used to test if a pointer value is NULL before accessing the data it points to



Bit Instructions

• Bit Test and Modify Instructions Four bit test instructions Each takes the position of the bit to be tested

Instruction Effect on the selected bitbt (Bit Test) No effectbts (Bit Test and Set) selected bit 1btr (Bit Test and Reset) selected bit 0btc selected bit NOT(selected bit)(Bit Test and Complement)



Bit Instructions (cont’d)

• All four instructions have the same format• We use bt to illustrate the format

bt operand,bit_pos operand is word or doubleword

» Can be in memory or a register bit_pos indicates the position of the bit to be tested

» Can be an immediate value or in a 16- or 32-bit register

• Instructions in this group affect only the carry flag» Other five flags are undefined following a bit test instruction



Bit Scan Instructions

• These instructions scan the operand for a 1 bit and return the bit position in a register

• Two instructionsbsf dest_reg,operand ;bit scan forwardbsr dest_reg,operand ;bit scan reverse» operand can be a word or doubleword in a register or

memory» dest_reg receives the bit position

– Must be a 16- or 32-bit register

Only ZF is updated (other five flags undefined)– ZF = 1 if all bits of operand are 0– ZF = 0 otherwise (position of first 1 bit in dest_reg)



Illustrative Examples

• Example 1 Multiplication using shift and add operations

» Multiplies two unsigned 8-bit numbers– Uses a loop that iterates 8 times

• Example 2 Same as Example 1 (efficient version)

» We loop only for the number of 1 bits– Uses bit test instructions

• Example 3 Conversion of octal to binary

Last slide